; Mode X (320x240, 256 colors) rectangle fill routine. Works on all
; VGAs. Uses fast approach that fans data out to up to four planes at
; once to draw up to four pixels at once. Fills up to but not
; including the column at EndX and the row at EndY. No clipping is
; performed.
; C near-callable as:
;    void FillRectangleX(int StartX, int StartY, int EndX, int EndY,
;	unsigned int PageBase, int Color);

SC_INDEX	equ	03c4h	;Sequence Controller Index
MAP_MASK	equ	02h	;index in SC of Map Mask register
SCREEN_SEG	equ	0a000h	;segment of display memory in mode X
SCREEN_WIDTH	equ	80	;width of screen in bytes from one scan line
				; to the next
parms	struc
	dw	2 dup (?)	;pushed BP and return address
StartX	dw	?		;X coordinate of upper left corner of rect
StartY	dw	?		;Y coordinate of upper left corner of rect
EndX	dw	?		;X coordinate of lower right corner of rect
				; (the row at EndX is not filled)
EndY	dw	?		;Y coordinate of lower right corner of rect
				; (the column at EndY is not filled)
PageBase dw	?		;base offset in display memory of page in
				; which to fill rectangle
Color	dw	?		;color in which to draw pixel
parms	ends

	.model	small
	.data
; Plane masks for clipping left and right edges of rectangle.
LeftClipPlaneMask	db	00fh,00eh,00ch,008h
RightClipPlaneMask	db	00fh,001h,003h,007h
	.code
	public	_FillRectangleX
_FillRectangleX proc	near
	push	bp		;preserve caller's stack frame
	mov	bp,sp		;point to local stack frame
	push	si		;preserve caller's register variables
	push	di

	cld
	mov	ax,SCREEN_WIDTH
	mul	[bp+StartY]	;offset in page of top rectangle scan line
	mov	di,[bp+StartX]
	shr	di,1		;X/4 = offset of first rectangle pixel in scan
	shr	di,1		; line
	add	di,ax		;offset of first rectangle pixel in page
	add	di,[bp+PageBase] ;offset of first rectangle pixel in
				; display memory
	mov	ax,SCREEN_SEG	;point ES:DI to the first rectangle
	mov	es,ax		; pixel's address
	mov	dx,SC_INDEX	;set the Sequence Controller Index to
	mov	al,MAP_MASK	; point to the Map Mask register
	out	dx,al
	inc	dx		;point DX to the SC Data register
	mov	si,[bp+StartX]
	and	si,0003h	;look up left edge plane mask
	mov	bh,LeftClipPlaneMask[si] ; to clip & put in BH
	mov	si,[bp+EndX]
	and	si,0003h	;look up right edge plane
	mov	bl,RightClipPlaneMask[si] ; mask to clip & put in BL

	mov	cx,[bp+EndX]	;calculate # of addresses across rect
	mov	si,[bp+StartX]
	cmp	cx,si
	jle	FillDone	;skip if 0 or negative width
	dec	cx
	and	si,not 011b
	sub	cx,si
	shr	cx,1
	shr	cx,1		;# of addresses across rectangle to fill - 1
	jnz	MasksSet	;there's more than one byte to draw
	and	bh,bl		;there's only one byte, so combine the left
				; and right edge clip masks
MasksSet:
	mov	si,[bp+EndY]
	sub	si,[bp+StartY]	;BX = height of rectangle
	jle	FillDone	;skip if 0 or negative height
	mov	ah,byte ptr [bp+Color] ;color with which to fill
	mov	bp,SCREEN_WIDTH ;stack frame isn't needed any more
	sub	bp,cx		;distance from end of one scan line to start
	dec	bp		; of next
FillRowsLoop:
	push	cx		;remember width in addresses - 1
	mov	al,bh		;put left-edge clip mask in AL
	out	dx,al		;set the left-edge plane (clip) mask
	mov	al,ah		;put color in AL
	stosb			;draw the left edge
	dec	cx		;count off left edge byte
	js	FillLoopBottom	;that's the only byte
	jz	DoRightEdge	;there are only two bytes
	mov	al,00fh		;middle addresses are drawn 4 pixels at a pop
	out	dx,al		;set the middle pixel mask to no clip
	mov	al,ah		;put color in AL
	rep	stosb		;draw the middle addresses four pixels apiece
DoRightEdge:
	mov	al,bl		;put right-edge clip mask in AL
	out	dx,al		;set the right-edge plane (clip) mask
	mov	al,ah		;put color in AL
	stosb			;draw the right edge
FillLoopBottom:
	add	di,bp		;point to the start of the next scan line of
				; the rectangle
	pop	cx		;retrieve width in addresses - 1
	dec	si		;count down scan lines
	jnz	FillRowsLoop
FillDone:
	pop	di		;restore caller's register variables
	pop	si
	pop	bp		;restore caller's stack frame
	ret
_FillRectangleX endp
	end